Overlay marks for reducing effect of bottom layer asymmetry

ABSTRACT

Methods of fabricating and using an overlay mark are provided. In some embodiments, the overlay mark includes an upper layer and a lower layer disposed below the upper layer. The lower layer includes a first plurality of compound gratings extending in a first direction and disposed in a first region of the overlay mark, each of the first plurality of compound gratings including one first element and at least two second elements disposed on one side of the first element, and a second plurality of compound gratings extending the first direction and disposed in a second region of the overlay mark, each of the second plurality of compound gratings including one third element and at least two fourth elements on one side of the third element. The first plurality of compound gratings is a mirror image of the second plurality of compound gratings.

PRIORITY DATA

This application is a divisional application of U.S. patent applicationSer. No. 16/295,510, filed Mar. 7, 2019, which claims the benefit ofU.S. Provisional Application No. 62/733,125, entitled “Overlay Marks forReducing Effect of Bottom Layer Asymmetry,” filed Sep. 19, 2018, each ofwhich is incorporated by reference herein in its entirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. Technological advances in IC materials and design have producedgenerations of ICs where each generation has smaller and more complexcircuits than the previous generation. However, these advances haveincreased the complexity of processing and manufacturing ICs and, forthese advances to be realized, similar developments in IC processing andmanufacturing are needed. In the course of integrated circuit evolution,functional density (i.e., the number of interconnected devices per chiparea) has generally increased while geometry size (i.e., the smallestcomponent (or line) that can be created using a fabrication process) hasdecreased.

Overlay marks have been used to measure the overlay or alignment betweenvarious layers of an IC. However, conventional overlay marks still haveshortcomings. For example, the measurement accuracy of a conventionaloverlay mark with an upper layer and a lower layer (sometimes referredto as a “bottom layer”) may be affected by asymmetry of the gratings inthe bottom layer. The asymmetry in the bottom gratings can induceadditional diffraction orders, resulting in reduced overlay accuracy.Therefore, while existing overlay marks and have been generally adequatefor their intended purposes, they have not been entirely satisfactory inevery aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a schematic view of a lithography system constructed inaccordance with some embodiments of the present disclosure.

FIG. 2 is a sectional view of a EUV mask constructed in accordance withsome embodiments of the present disclosure.

FIG. 3 illustrates a simplified fragmentary cross-sectional view of anoverlay mark 100 in accordance with some embodiments of the presentdisclosure.

FIG. 4A illustrates fragmentary cross-sectional view of an upper layer1400 and a lower layer 1300 in accordance with some embodiments of thepresent disclosure.

FIG. 4B illustrates fragmentary cross-sectional view of an upper layer1600 and a lower layer 1500 in accordance with some embodiments of thepresent disclosure.

FIG. 5 illustrates a top view of an embodiment of an overlay mark on asubstrate in accordance with some embodiments of the present disclosure.

FIG. 6 illustrates a top view of another embodiment of an overlay markon a substrate in accordance with some embodiments of the presentdisclosure.

FIG. 7 is a flowchart illustrating the process flow associated with theoverlay marks in accordance with some embodiments of the presentdisclosure

FIG. 8 illustrates a fragmentary top view of mandrels for forming abottom layer of an overlay mark on a substrate in accordance with someembodiments of the present disclosure.

FIG. 9A illustrates a fragmentary cross-sectional view of the mandrelfeatures in FIG. 8, according to embodiments of the present disclosure.

FIG. 9B illustrates a fragmentary cross-sectional view of spacermaterial deposited over the mandrel features in FIG. 9A, according toembodiments of the present disclosure.

FIG. 9C illustrates a fragmentary cross-sectional view of planarizedspacers and mandrel features on a substrate, according to embodiments ofthe present disclosure.

FIG. 9D illustrates a fragmentary cross-sectional view of spacers on asubstrate, according to embodiments of the present disclosure.

FIGS. 10 and 11 are flowcharts illustrating methods of semiconductorfabrication associated with the overlay marks in accordance with someembodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

To ensure accurate alignment (also referred to as overlay) between thevarious layers in a semiconductor device during a semiconductorfabrication process, overlay marks (or alignment marks) are used tomeasure the alignment between the layers. However, conventional overlaymarks may have shortcomings. For example, lower layers of conventionaloverlay marks can have asymmetric gratings, resulting in overlayinaccuracy.

To overcome the problems discussed above, the present disclosureprovides embodiments of overlay marks that can reduce overlay inaccuracyresulting from bottom grating asymmetry. The various aspects of thepresent disclosure will be discussed below in greater detail withreference to FIGS. 1-9D. First, a EUV lithography system will bediscussed below with reference to FIGS. 1-3 as an example lithographycontext in which the overlay mark of the present disclosure may be used,although it is understood that the overlay mark discussed herein may beused for other types of non-EUV lithography contexts too. Next, thedetails of the overlay mark according to embodiments of the presentdisclosure are discussed with reference to FIGS. 4A-9D.

FIG. 1 is a schematic view diagram of a EUV lithography system 10,constructed in accordance with some embodiments. The EUV lithographysystem 10 may also be generically referred to as a scanner that isconfigured to perform lithography exposure processes with respectiveradiation source and exposure mode. The EUV lithography system 10 isdesigned to expose a photoresist layer by EUV light or EUV radiation.The photoresist layer is a material sensitive to the EUV light. The EUVlithography system 10 employs a radiation source 12 to generate EUVlight, such as EUV light having a wavelength ranging between about 1 nmand about 100 nm. In one particular example, the radiation source 12generates a EUV light with a wavelength centered at about 13.5 nm.Accordingly, the radiation source 12 is also referred to as EUVradiation source 12.

The lithography system 10 also employs an illuminator 14. In variousembodiments, the illuminator 14 includes various refractive opticcomponents, such as a single lens or a lens system having multiplelenses (zone plates) or alternatively reflective optics (for EUVlithography system), such as a single mirror or a mirror system havingmultiple mirrors in order to direct light from the radiation source 12onto a mask stage 16, particularly to a mask 18 secured on the maskstage 16. In the present embodiment where the radiation source 12generates light in the EUV wavelength range, the illuminator 14 employsreflective optics. In some embodiments, the illuminator 14 includes adipole illumination component.

In some embodiments, the illuminator 14 is operable to configure themirrors to provide a proper illumination to the mask 18. In one example,the mirrors of the illuminator 14 are switchable to reflect EUV light todifferent illumination positions. In some embodiment, a stage prior tothe illuminator 14 may additionally include other switchable mirrorsthat are controllable to direct the EUV light to different illuminationpositions with the mirrors of the illuminator 14. In some embodiments,the illuminator 14 is configured to provide an on-axis illumination(ONI) to the mask 18. In an example, a disk illuminator 14 with partialcoherence σ being at most 0.3 is employed. In some other embodiments,the illuminator 14 is configured to provide an off-axis illumination(OAI) to the mask 18. In an example, the illuminator 14 is a dipoleilluminator. The dipole illuminator has a partial coherence σ of at most0.3 in some embodiments.

The lithography system 10 also includes a mask stage 16 configured tosecure a mask 18. In some embodiments, the mask stage 16 includes anelectrostatic chuck (e-chuck) to secure the mask 18. This is because gasmolecules absorb EUV light, and the lithography system for the EUVlithography patterning is maintained in a vacuum environment to avoidthe EUV intensity loss. In the disclosure, the terms of mask, photomask,and reticle are used interchangeably to refer to the same item.

In the present embodiment, the lithography system 10 is a EUVlithography system, and the mask 18 is a reflective mask. One exemplarystructure of the mask 18 is provided for illustration. The mask 18includes a substrate with a suitable material, such as a low thermalexpansion material (LTEM) or fused quartz. In various examples, the LTEMincludes TiO₂ doped SiO₂, or other suitable materials with low thermalexpansion. In some embodiments, the LTEM includes 5%-20% by weight TiO₂and has a thermal coefficient of expansion lower than about 1.0×10−6/°C. For example, in some embodiments, the TiO₂ doped SiO₂ material of theLTEM has a coefficient thermal expansion such that it varies by lessthan 60 parts-per-billion for every 1 degree Celsius of temperaturechange. Of course, other suitable materials having thermal coefficientof expansion that is equal to or less than TiO₂ doped SiO₂ may also beused.

The mask 18 also includes a reflective multilayer (ML) deposited on thesubstrate. The ML includes a plurality of film pairs, such asmolybdenum-silicon (Mo/Si) film pairs (e.g., a layer of molybdenum aboveor below a layer of silicon in each film pair). Alternatively, the MLmay include molybdenum-beryllium (Mo/Be) film pairs, or other suitablematerials that are configurable to highly reflect the EUV light.

The mask 18 may further include a capping layer, such as ruthenium (Ru),disposed on the ML for protection. The mask 18 further includes anabsorption layer deposited over the ML. The absorption layer ispatterned to define a layer of an integrated circuit (IC).Alternatively, another reflective layer may be deposited over the ML andis patterned to define a layer of an integrated circuit, thereby forminga EUV phase shift mask.

The lithography system 10 also includes a projection optics module (orprojection optics box (POB) 20 for imaging the pattern of the mask 18 onto a target 26 secured on a substrate stage 28 of the lithography system10. The POB 20 has refractive optics (such as for UV lithography system)or alternatively reflective optics (such as for EUV lithography system)in various embodiments. The light directed from the mask 18, diffractedinto various diffraction orders and carrying the image of the patterndefined on the mask, is collected by the POB 20. The POB 20 may includea magnification of less than one (thereby the size of the “image” on atarget (such as target 26 discussed below) is smaller than the size ofthe corresponding “object” on the mask). The illuminator 14 and the POB20 are collectively referred to as an optical module of the lithographysystem 10.

The lithography system 10 also includes a pupil phase modulator 22 tomodulate optical phase of the light directed from the mask 18 so thatthe light has a phase distribution on a projection pupil plane 24. Inthe optical module, there is a plane with field distributioncorresponding to Fourier Transform of the object (the mask 18 in thepresent case). This plane is referred to as projection pupil plane. Thepupil phase modulator 22 provides a mechanism to modulate the opticalphase of the light on the projection pupil plane 24. In someembodiments, the pupil phase modulator 22 includes a mechanism to tunethe reflective mirrors of the POB 20 for phase modulation. For example,the mirrors of the POB 20 are switchable and are controlled to reflectthe EUV light, thereby modulating the phase of the light through the POB20.

In some embodiments, the pupil phase modulator 22 utilizes a pupilfilter placed on the projection pupil plane. A pupil filter filters outspecific spatial frequency components of the EUV light from the mask 18.Particularly, the pupil filter is a phase pupil filter that functions tomodulate phase distribution of the light directed through the POB 20.However, utilizing a phase pupil filter is limited in some lithographysystem (such as an EUV lithography system) since all materials absorbEUV light.

As discussed above, the lithography system 10 also includes thesubstrate stage 28 to secure a target 26 to be patterned, such as asemiconductor substrate. In the present embodiment, the semiconductorsubstrate is a semiconductor substrate, such as a silicon substrate orother type of substrate. The target 26 is coated with the resist layersensitive to the radiation beam, such as EUV light in the presentembodiment. Various components including those described above areintegrated together and are operable to perform lithography exposingprocesses. The lithography system 10 may further include other modulesor be integrated with (or be coupled with) other modules.

The mask 18 and the method making the same are further described inaccordance with some embodiments. In some embodiments, the maskfabrication process includes two operations: a blank mask fabricationprocess and a mask patterning process. During the blank mask fabricationprocess, a blank mask is formed by deposing suitable layers (e.g.,reflective multiple layers) on a suitable substrate. The blank mask isthen patterned during the mask patterning process to achieve a desireddesign of a layer of an integrated circuit (IC). The patterned mask isthen used to transfer circuit patterns (e.g., the design of a layer ofan IC) onto a semiconductor substrate. The patterns can be transferredover and over onto multiple substrates through various lithographyprocesses. A set of masks is used to construct a complete IC.

The mask 18 includes a suitable structure, such as a binary intensitymask (BIM) and phase-shifting mask (PSM) in various embodiments. Anexample BIM includes absorptive regions (also referred to as opaqueregions) and reflective regions, patterned to define an IC pattern to betransferred to the target. In the opaque regions, an absorber ispresent, and an incident light is almost fully absorbed by the absorber.In the reflective regions, the absorber is removed and the incidentlight is diffracted by a multilayer (ML). The PSM can be an attenuatedPSM (AttPSM) or an alternating PSM (AltPSM). An exemplary PSM includes afirst reflective layer (such as a reflective ML) and a second reflectivelayer patterned according to an IC pattern. In some examples, an AttPSMusually has a reflectivity of 2%-15% from its absorber, while an AltPSMusually has a reflectivity of larger than 50% from its absorber.

One example of the mask 18 is shown in FIG. 2. The mask 18 in theillustrated embodiment is a EUV mask, and includes a substrate 30 madeof a LTEM. The LTEM material may include TiO₂ doped SiO₂, and/or otherlow thermal expansion materials known in the art. In some embodiments, aconductive layer 32 is additionally disposed under on the backside ofthe LTEM substrate 30 for the electrostatic chucking purpose. In oneexample, the conductive layer 32 includes chromium nitride (CrN). Inother embodiments, other suitable compositions are possible, such as atantalum-containing material.

The EUV mask 18 includes a reflective multilayer (ML) structure 34disposed over the LTEM substrate 30. The ML structure 34 may be selectedsuch that it provides a high reflectivity to a selected radiationtype/wavelength. The ML structure 34 includes a plurality of film pairs,such as Mo/Si film pairs (e.g., a layer of molybdenum above or below alayer of silicon in each film pair). Alternatively, the ML structure 34may include Mo/Be film pairs, or any materials with refractive indexdifference being highly reflective at EUV wavelengths.

Still referring to FIG. 2, the EUV mask 18 also includes a capping layer36 disposed over the ML structure 34 to prevent oxidation of the ML. Inone embodiment, the capping layer 36 includes silicon with a thicknessranging from about 4 nm to about 7 nm. The EUV mask 18 may furtherinclude a buffer layer 38 disposed above the capping layer 36 to serveas an etching-stop layer in a patterning or repairing process of anabsorption layer, which will be described later. The buffer layer 38 hasdifferent etching characteristics from the absorption layer disposedthereabove. The buffer layer 38 includes ruthenium (Ru), Ru compoundssuch as RuB, RuSi, chromium (Cr), chromium oxide, and chromium nitridein various examples.

The EUV mask 18 also includes an absorber layer 40 (also referred to asan absorption layer) formed over the buffer layer 38. In someembodiments, the absorber layer 40 absorbs the EUV radiation directedonto the mask. In various embodiments, the absorber layer may be made oftantalum boron nitride (TaBN), tantalum boron oxide (TaBO), or chromium(Cr), Radium (Ra), or a suitable oxide or nitride (or alloy) of one ormore of the following materials: Actium, Radium, Tellurium, Zinc,Copper, and Aluminum.

The EUV lithography system discussed above in FIGS. 1-2 is merely anexample lithography system for which overlay marks can be used. However,the overlay marks of the present disclosure may be used for other typesof lithography systems having different light sources. The overlay marksof the present disclosure will now be discussed below in more detail.

FIG. 3 illustrates a simplified fragmentary cross-sectional side view ofan overlay mark 100. The overlay mark 100 includes an upper layer 100Aand a lower layer 100B. In some embodiments, the upper layer 100Aincludes a patterned photoresist layer, and the lower layer 100Bincludes a patterned spacer layer on a substrate. In other embodiments,the upper layer 100A and the lower layer 100B may include differentpatterned layers on a substrate.

The upper layer 100A and the lower layer 100B each include a pluralityof patterned components, also referred to as gratings. For example, theupper layer 100A includes a plurality of gratings 110A, and the lowerlayer 100B includes a plurality of gratings 110B. The gratings 110A and110B are elongated features that extend in a certain direction, forexample in a direction orthogonal to the cross-section in which thecross-sectional view of FIG. 3 is taken. In some embodiments, thegratings 110A are periodically distributed, and/or the gratings 110B areperiodically distributed. In other words, the gratings 110A areseparated from one another by a constant spacing, and the gratings 110Bare separated from one another by a constant spacing.

An overlay between the upper layer 100A and the lower layer 100B may bemeasured by light diffraction. For example, in response to incidentlight projected onto the overlay mark 100, different orders ofdiffracted light may be produced as a result. In FIG. 3, a 0th orderdiffracted light is shown as I₀, a +1 order diffracted light is shown asI₊₁, and a −1 order diffracted light is shown as I⁻¹. The intensities ofthe various diffracted orders of light may be measured by an opticalmeasurement tool. In some embodiments, the optical measurement toolincludes a scatterometry machine. In some other embodiments, the opticalmeasurement tool includes a diffractometry machine. It is understoodthat the optical measurement tool may also be configured to generate theincident light in some embodiments. Based on the measured I₊₁ and I⁻¹data, asymmetry information (As) associated with the overlay mark 100can be defined as =I₊₁−I⁻¹. The asymmetry information is used todetermine overlay, as discussed in more detail below.

Referring now to FIG. 4A, shown therein is fragmentary cross-sectionalview of an upper layer 1400 and a lower layer 1300 of an overlay mark202 according to some embodiments of the present disclosure. In someembodiments, the overlay mark 202 includes region 1 and region 2. Insome implementations, the upper layer 1400 and the lower layer 1300 maybe two different layers of the overlay mark 202 on a substrate, such asa photomask. The lower layer 1300 includes a plurality of compoundgratings 130 in region 1 and a plurality of compound gratings 132 inregion 2. The plurality of compound gratings 130 extend along the ydirection (into and out of the cross-sectional plane). In someimplementations, the plurality of compound gratings 130 in the lowerlayer 1300 includes one elongated element 130A having a width W1, aplurality of elongated elements 130B each having a width W2, oneelongated element 130C having a width W3, and one elongated element 130Dwith a width W4, where the widths are measured in an X-directionperpendicular to the Y-direction. In the implementations represented byFIG. 4A, the width W2 is smaller than each of the width W1, the width W3and the width W4. The width W1, the width W3 and the Width W4 can bedifferent from one another due to intentional loading effectdifferential introduced by different widths of the mandrels used to formthe elongated elements. In one non-limiting example, the width W4 isgreater than the width W3. In some embodiments, the plurality ofcompound gratings 130 in region 1 of the lower layer 1300 includes a gap130E between the elongated element 130C and the elongated element 130D.In some instances, the gap 130E includes a width W5, and W5 isrepresents a width of a removed mandrel that is used to form theelongated element 130C and the elongated element 130D. In someembodiments, the plurality of elongated elements 130B is periodicallydisposed at a pitch P, and each of the plurality of elongated elements130B is separated from one another by a constant spacing. The constantspacing is smaller than the width W5. In some instances, the pluralityof elongated elements 130B includes 2 to 15 elongated elements, forexample 4 to 12 elongated elements. While the embodiments shown in FIG.4A include pluralities of compound gratings 130 and 132 extending alongthe Y direction, the pluralities of compound gratings may be arranged toextend along the X direction. It is also understood that the region 1may have multiple groups of the plurality of compound gratings 130. Insome embodiments, these groups of the plurality of compound gratings 130are periodically repeated.

The plurality of compound gratings 132 in region 2 is a mirror image ofthe plurality of compound gratings 130 in region 1 with respect to theborderline 210 between region 1 and region 2. The plurality of compoundgratings 132 in the lower layer 1300 includes one elongated element 132Ahaving the width W1, a plurality of elongated elements 132B each havingthe width W1, one elongated element 132C having the width W3, and oneelongated element 132D with the width W4. Similarly, in some instances,the width W1 is greater than the width W4, the width W4 is greater thanthe width W3, and the width W3 is greater than the width W2. In someembodiments, the plurality of elongated elements 132B is periodicallydisposed at the pitch P, and each of the plurality of elongated elements132B is separated from one another by the constant spacing. In someembodiments, the plurality of compound gratings 132 in region 1 of thelower layer 1300 includes a gap 132E between the elongated element 132Cand the elongated element 132D. In some instances, the gap 132E includesthe width W5, and the width W5 is greater than W1. The width W5 isgreater than the constant spacing. In some embodiments, the plurality ofelongated elements 132B includes 2 to 15 elongated elements, for example4 to 12 elongated elements. It is also understood that the region 2 mayhave multiple groups of the plurality of compound gratings 132. In someembodiments, these groups of the plurality of compound gratings 132 areperiodically repeated.

In the embodiments represented by FIG. 4A, the upper layer 1400 includesa plurality of gratings 140 in region 1 and a plurality of gratings 142in region 2. Both the plurality of gratings 140 and the plurality ofgratings 142 extend along the Y direction (into and out of thecross-sectional plane) as well. In some implementations, the pluralityof gratings 140 in region 1 includes elongated elements 140A disposed atthe pitch P, and the plurality of gratings 142 in region 2 includeselongated elements 142A disposed at the same pitch P. The plurality ofgratings 140 and the plurality of gratings 142 are identical and equallypitched. Each of the gratings in the plurality of gratings 140 and inthe plurality of gratings 142 has the width W2. In some embodiments asshown in FIG. 4A, the plurality of gratings 140 is disposed above andover the plurality of compound gratings 130, and the plurality ofgratings 142 is disposed above and over the plurality of compoundgratings 132.

A known bias may be introduced between the upper layer 1400 and thelower layer 1300. For example, although the plurality of gratings 140 inregion 1 of the upper layer 1400 shares the same pitch P and the samewidth W2 with the plurality of elongated elements 130B in the lowerlayer 1300, the plurality of gratings 140 is shifted by a distance dwith respect to the plurality of elongated elements 130B along the −Xdirection (e.g., shifted to the “left” as shown in FIG. 4A). This shiftin region 1 can be referred to as bias −d. Similarly, the plurality ofgratings 142 in region 2 of the upper layer 1400 is shifted by adistance d with respect to the plurality of elongated elements 132Balong the +X direction (e.g., shifted to the “right” as shown in FIG.4A). This shift in region 2 can be referred to as bias +d. The bias −din region 1 and the bias +d in region 2 may be intentionally configuredor implemented as part of the design of the photomask.

As shown in the embodiments represented by FIG. 4A, out of the elongatedelements of each of the plurality of compound gratings 130, theelongated element 130A with the width W1 is the closest to theborderline 210, and the elongated element 130D with the width W4 is thefarthest away from the borderline 210. The plurality of compoundgratings 132 in region 2, being the mirror image of the plurality ofcompound gratings 130 in region 1, includes a symmetric arrangement. Theelongated element 132A with the width W1 is the closest to theborderline 210, and the elongated element 130D with the width W4 is thefarthest away from the borderline 210. In some instances, the pluralityof compound gratings 130 in region 1 can be referred to as “normal”gratings, and the plurality of compound gratings 132 in region 2 can bereferred to as “inverse” gratings. Taking into consideration of theknown bias introduced into the overlay mark 202, region 1 can bereferred to “−d normal” and region 2 can be referred to as “+d normal.”

Referring now to FIG. 4B, shown therein is another overlay mark 204.Similar to the embodiment represented by FIG. 4A, along the Z direction,the overlay mark 204 includes a lower layer 1500 and an upper layer1600; and along the X direction, the overlay mark 204 includes a region1 and a region 2. In some embodiments, the upper layer 1600 of theoverlay mark 204 is substantially identical to the upper layer 1400 ofthe overlay mark 202. In region 1 of the lower layer 1500 is a pluralityof compound gratings 150. In region 2 of the lower layer 1500 is aplurality of compound gratings 152. In some implementations, theplurality of compound gratings 152 in region 2 is substantiallyidentical to the plurality of compound gratings 130 of the overlay mark202, and the plurality of compound gratings 150 in region 1 issubstantially identical to the plurality of compound gratings 132 of theoverlay mark 202. That is, the plurality of compound gratings 152 inregion 2 may be referred to as “normal” gratings and the plurality ofcompound gratings 150 in region 1 may be referred to as “inverse”gratings. In some implementations, each of the plurality of compoundgrating 150 includes one elongated element 150A, a plurality ofelongated elements 150B, one elongated element 150C, and one elongatedelement 150D. In some embodiments, each of the plurality of compoundgratings 152 includes one elongated element 152A, a plurality ofelongated elements 152B, one elongated element 152C, and one elongatedelement 152D. Out of the elongated elements 150A-150D, the elongatedelement 150D is the closest to the borderline 220 between region 1 andregion 2, and the elongated element 150A is the farthest away from theborderline 220. Out of the elongated elements 152A-152D, the elongatedelement 152D is the closest to the borderline 220 between region 1 andregion 2, and elongated element 152A is the farthest away from theborderline 220.

In some embodiments, similar to the overlay mark 202, the plurality ofgratings 160 and the plurality of gratings 162 in the upper layer 1600of overlay mark 204 each include the pitch P. In a similar fashion, thepluralities of elongated elements 150B and 152B include the pitch P aswell. The known bias d can be introduced between the upper layer 1600and the lower layer 1500 in regions 1 and 2. In region 1, the pluralityof gratings 160 is disposed above the plurality of compound gratings 150and is shifted in the −X direction by a distance d. In region 2, Theplurality of gratings 162 is above the plurality of compound gratings152 and is shifted in the +X direction by a distance d. Viewing theexemplary overlay mark 202 in FIG. 4A and overlay mark 204 in FIG. 4B asa whole, region 1 of FIG. 4A can be referred to as “−d normal,” region 2of FIG. 4A can be referred to as “+d inverse,” region 1 of FIG. 4B canbe referred to as “−d inverse” and region 2 of FIG. 4B can be referredto as “+d normal.” The plurality of elongated elements 150B includes 2to 15 elongated elements, for example 4 to 12 elongated segments. Theplurality of elongated elements 152B includes 2 to 15 elongatedelements, for example 4 to 12 elongated elements.

Embodiments of the present disclosure provide advantages. Taking theoverlay mark 202 in FIG. 4A as an example, the overlay information ofregion 1 can be described as (OVL −OVL_(BGA)). In the expression, theplurality of elongated elements 130B in region 1 of the lower layer 1300and the compound gratings 140 in region 1 of the upper layer 1400contribute to the overlay term OVL and provide alignment information.The elongated element 130A, the elongated element 130C and the elongatedelement 130D contribute to the additional bottom grating asymmetry (BGA)error term −OVL_(BGA). The overlay information of region 2 can bedescribed as (OVL+OVL_(BGA)). In the expression, the plurality ofelongated elements 132B in region 2 of the lower layer 1300 and thecompound gratings 142 in region 2 of the upper layer 1400 contribute tothe overlay term OVL and provide alignment information. The elongatedelement 132A, the elongated element 132C and the elongated element 132Dcontribute to the additional BGA error term +OVL_(BGA). Viewing theoverlay mark 202 as a whole, the overlay information of the overlay mark202 can be expressed as (OVL−OVL_(BGA)+OVL+OVL_(BGA))/2. Because the BGAerror term from region 1 and the BGA error term from region 2 aresubstantially equal in magnitude and opposite in polarity, the BGA errorterms can be canceled out, and the above expression(OVL−OVL_(BGA)+OVL+OVL_(BGA))/2 can be simplified as OVL. Thedesigned-in BGA error terms for region 1 and region 2 reduce theasymmetry in the lower layer (or referred to as the bottom layer),improving the overlay accuracy.

In some instances, the design of the certain compound gratings in thelower layer (or bottom layer) can have varying densities therein. Takingthe compound gratings 132 as an example, due to the presence of the gap132E, the gratings on the left-hand side of the compound gratings 132are denser than the right-hand side thereof. This design of compoundgratings 132 includes a wider mandrel to form the gap and a plurality ofnarrower mandrels to form the denser side of the compound gratings 132.The difference in mandrel density can introduce different loading andcreate unevenness or imperfection in the compound gratings 132. Thecompound gratings 130 are a mirror image of the compound gratings 132.Because formation of the compound gratings 130 includes a mirror imageof the mandrels used to form the compound gratings 132, the unevennessor imperfection in the compound grating 130 is likely a mirror image ofthe unevenness or imperfection in the compound grating 132. This mirrorimaging allows the error terms in region 1 and region 2 to cancel eachother out, yielding better alignment accuracy. In other words, althoughimperfections may be caused by the different densities of the compoundgratings 132, these imperfections may be obviated by the fact that thecompound gratings 130 are designed as a mirror image of the compoundgratings 132.

Referring now to FIG. 5, shown therein is a combination overlay mark300. The overlay mark 300 includes a region I and a region II. In someembodiments, region I and region II are adjacent to (e.g., contiguous toeach another) or near one another. In some embodiments, region I andregion II are spaced apart. In some embodiments represented by FIG. 5,region I of the overlay mark 300 includes an area A with a +d normaloverlay mark 301, an area A′ with a −d normal overlay mark 302, an areaB with a +d inverse overlay mark 311, and an area B′ with a −d inverseoverlay mark 312. Region II of the overlay mark 300 includes an area Cwith a +d normal overlay mark 321, an area C's with a −d normal overlaymark 322, an area D with a +d inverse overlay mark 331, and an area D′with a −d inverse overlay mark 332. In some embodiments, the elongatedelements and gratings in areas A, A′, B, and B′ extend along the Xdirection, and the elongated elements and gratings in areas C, C′, D,and D′ extend along the Y direction. In alternative embodiments, theelongated elements and gratings in areas A, A′, B, and B′ extend alongthe Y direction, and the elongated elements and gratings in areas C, C′,D, and D′ extend along the X direction.

FIG. 6 is another embodiment of a combination overlay mark 400. Theoverlay mark 400 includes an area A with a +d normal overlay mark 401,an area A′ with a −d normal overlay mark 402, an area B with a +dinverse overlay mark 411, an area B′ with a −d inverse overlay mark 412,an area C with a +d normal overlay mark 421, an area C′ with a −d normaloverlay mark 422, an area D with a +d inverse overlay mark 431, and anarea D′ with a −d inverse overlay mark 432. In some embodimentsrepresented by FIG. 6, the elongated elements and gratings in areas A,A′, B, and B′ extend along the X direction, and the elongated elementsand gratings in areas C, C′, D, and D′ extend along the Y direction. Inalternative embodiments, the elongated elements and gratings in areas A,A′, B, and B′ extend along the Y direction, and the elongated elementsand gratings in areas C, C′, D, and D′ extend along the X direction.

It is noted that the mirror image compound gratings pairs do not have tobe aligned with and adjacent to one another. In the embodimentsrepresented by FIGS. 4A and 4B, the plurality of compound gratings 130in region 1 is the mirror image of the plurality of compound gratings132 in region 2. Region 1 of the overlay 202 is aligned with andadjacent to region 2. In the embodiments represented by FIGS. 5 and 6,area A is a mirror image of area B′, area B is a mirror image of areaA′, area C is a mirror image of area D′, area D is a mirror image ofarea C′.

The overlay marks disclosed herein, including the upper layers and lowerlayers of overlay marks 100, 202, 204, 300 and 400, can be fabricated inany areas of an IC devices. In some embodiments, these overlay marks canbe fabricated in scribe lines or scribe areas, which are subject tocutting in singulation processes. In these embodiments, at least aportion of the overlay marks in a singulated die is damaged, leavingbehind some remnant overlay marks. In some alternative embodiments,these overlay marks can be fabricated in device areas (i.e. outside ofthe scribe lines or scribe areas), which are not subject to cutting insingulation processes. In these alternative embodiments, these overlaymarks can survive the singulation process and remain intact in a finalIC device. Both the intact overlay marks and remnant overlay marksaccording to the present disclosure can demonstrate a portion of overlaymarks being a mirror image or another portion of the overlay marks.

Referring now to FIG. 7, illustrated therein is a flowchart of a method500 of fabricating an overlay mark on a substrate. The method 500 ismerely an example, and is not intended to limit the present disclosurebeyond what is explicitly recited in the claims. Additional operationscan be provided before, during, and after the method 500, and someoperations described can be replaced, eliminated, or moved around foradditional embodiments of the method. Exemplary operations of the method500 will be described below with reference to FIGS. 8 and 9A-9D.

Referring now to FIGS. 7-8, at operation 510 of the method 500, aplurality of mandrel features 601A of an overlay mark 600 is formed inregion 1, and a plurality of mandrel features 601B of the overlay mark600 is formed in region 2 on a substrate 608. The mandrel features canbe fabricated with conventional mandrel forming processes. Note thatregion 1 has multiple sets or groups of the plurality of mandrelfeatures 601A, and region 2 has multiple sets or groups of the pluralityof mandrel features 601B. The pluralities of mandrel features 601A and601B extend along the Y direction shown in FIG. 8. In one embodiment,each of the plurality of mandrel features 601A includes one elongatedelement 604A and a plurality of elongated elements 602A. The pluralityof mandrel features 601A is disposed at a pitch 605. For example, thepitch 605 (or a distance measured in the X-direction) separates onegroup of the mandrel features 601A from its nearest group of mandrelfeatures 601A, as shown in FIG. 8. The elongated element 604A includes awidth W6 along the X direction, and each of the plurality of elongatedelements 602A is equally spaced and includes a width W7. In someembodiments represented by FIG. 8, the width W6 is greater than thewidth W7. In some embodiments, the width W6 is at least twice as thewidth W7 to ensure meaningful loading effect brought about by the largerwidth W6. In some implementations, the plurality of elongated elements602A includes 2 to 15 elongated elements with the width W7. Theplurality of mandrel features 601B in region 2 is a mirror image of theplurality of mandrel features 601A in region 1 with respect to theborderline 610 between regions 1 and 2. Consequently, the plurality ofmandrel features 601B is also disposed at the same pitch 605. Each ofthe plurality of mandrel features 601B includes one elongated element604B and a plurality of elongated elements 602B. In the embodimentswhere the elongated element 604A in region 1 has the width W6 and eachof the plurality of elongated elements 602A in region 1 has the widthW7, the elongated element 604B includes the width W6, and each of theplurality of elongated elements 602B is equally spaced and includes thewidth W7. FIG. 9A illustrates the Y-direction cross-sectional view ofthe mandrel features in FIG. 8. The greater width (at least twice of W7)of W6 can give rise to loading effects and result in intentionallyintroduced unevenness and imperfection in the overlay mark. Because theplurality of mandrel features 601B in region 2 is a mirror image of theplurality of mandrel features 601A in region 1, the unevenness andimperfection in regions 1 and 2 can cancel out and improve the overlayaccuracy.

At operations 520, 530 and 540 of the method 500, spacers are formedover sidewalls of the plurality of mandrel features 601A and theplurality of mandrel features 601B. Referring now to FIG. 9B, atoperation 520 of the method 500, a spacer layer 700 is deposited overthe plurality of mandrel features 601A and the plurality of mandrelfeatures 601B, including over the space between elongated elements.

Reference is now made to FIG. 9C. At operation 530, the deposited spacerlayer 700 is planarized to expose the plurality of mandrel features 601Aand the plurality of mandrel features 601B.

Referring to FIG. 9D, at operation 540, the plurality of mandrelfeatures 601A and the plurality of mandrel features 601B are removed,leaving behind a plurality of spacers 800 in region 1 and a plurality ofspacers 900 in region 2. The plurality of spacers 800 may be referred toas a plurality of compound gratings 800 (e.g., as an embodiment of thecompound gratings 130 or 150 discussed above with reference to FIGS.4A-4B). The plurality of spacers 900 may be referred to as a pluralityof compound gratings 900 (e.g., as an embodiment of the compoundgratings 132 or 152 discussed above with reference to FIGS. 4A-4B). Insome implementations, each of the plurality of compound grating 800includes one elongated element 800A, one elongated element 800B, aplurality of elongated elements 800C, and one elongated element 800D. Insome embodiments, each of the plurality of compound gratings 900includes one elongated element 900A, one elongated element 900B, aplurality of elongated elements 900C, and one elongated element 900D. Inthe implementations represented by FIG. 9D, as the plurality of mandrelfeatures 601B is a mirror image of the plurality of mandrel features601A with respect to the borderline 610, the plurality of compoundgratings 900 is a mirror image of the plurality of compound gratings 800with respect to the borderline 610 as well. In some implementations,each of the elongated elements 800A and 900A has the width W4, each ofthe elongated elements 800B and 900B has the width W3, each of theelongated elements 800C and 900C has the width W2, and each of theelongated elements 800D and 900D has the width W1. In theimplementations represented by FIG. 4A, the width W2 is smaller thaneach of the width W1, the width W3 and the width W4. The width W1, thewidth W3 and the Width W4 can be different from one another due tointentional loading effect differential introduced by different widthsof the mandrels used to form the elongated elements. In one non-limitingexample, the width W4 is greater than the width W3.

FIG. 10 is a flowchart illustrating a method 1000 of semiconductorfabrication according to aspects of the present disclosure. The method1000 includes a step 1002 of patterning an overlay mark on a substrate.The overlay mark includes an upper layer; and a lower layer disposedbelow the upper layer. The lower layer can include a first plurality ofcompound gratings and a second plurality of compound gratings. The firstplurality of compound gratings extends in a first direction and isdisposed in a first region of the overlay mark. Each of the firstplurality of compound gratings can include one first element and atleast two second elements disposed on one side of the first element. Thesecond plurality of compound gratings extends in the first direction andis disposed in a second region of the overlay mark. Each of the secondplurality of compound gratings can include one third element and atleast two fourth elements disposed on one side of the third element.Each of the first element and the third element has a first width alonga second direction perpendicular to the first direction. Each of thesecond elements and the fourth elements has a second width along thesecond direction. The second width is smaller than the first width. Thefirst plurality of compound gratings is a mirror image of the secondplurality of compound gratings. The method 1000 further includes a step1004 of performing one or more semiconductor manufacturing processesusing the overlay mark.

In some embodiments, each of the first plurality of compound gratingsmay further include one fifth element and the at least two secondelements are disposed between the first element and the fifth element.In some embodiments, each the first plurality of compound gratings mayfurther include one sixth element disposed between the at least twofourth elements and the fifth element. In some implementations, each ofthe first plurality of compound gratings may further include one gapdisposed between the sixth element and the fifth element. In someimplementations, the fifth element has a third width and the sixthelement has a fourth width. Each of the third width and the fourth widthis greater than the second width. In some embodiments, the at least twosecond elements comprise 4 to 12 second elements. In some embodiments,the upper layer may include a third plurality of gratings and the thirdplurality of gratings may be shifted with respect to the at least twosecond elements in the second direction. In some instances, the upperlayer may further include a fourth plurality of gratings and the fourthplurality of gratings is shifted with respect to the at least two fourthelements in the second direction. In some embodiments, the first regionof the overlay mark is adjacent to and aligned with the second region ofthe overlay mark along the second direction.

It is understood that additional processes may be performed before,during, or after the steps 1002-1004 of the method 1000. For reasons ofsimplicity, additional steps are not discussed herein in detail.

FIG. 11 is a flowchart illustrating a method 1100 of semiconductorfabrication according to aspects of the present disclosure. The method1100 includes a step 1102 of patterning an overlay mark on a substrate.The overlay mark includes an upper layer that includes a plurality ofgratings extending in a first direction, and a lower layer disposedbelow the upper layer. The lower layer may include a first plurality ofcompound gratings extending in the first direction and disposed in afirst region of the overlay mark and a second plurality of compoundgratings extending the first direction and disposed in a second regionof the overlay mark. Each of the first plurality of compound gratingsmay include one first element, one second element, and at least twothird elements disposed between the first element and the secondelement. The first element has a first width along a second directionperpendicular to the first direction and each of the at least two thirdelements has a second width along the second direction. The second widthis smaller than the first width. The second plurality of compoundgratings is a mirror image of the first plurality of compound gratings.The plurality of gratings is shifted with respect to the at least twothird elements in the second direction. The method 1100 further includesa step 1104 of performing one or more semiconductor manufacturingprocesses using the overlay mark.

In some embodiments, a portion of the second plurality of compoundgratings may be shifted with respect to the plurality of gratings in thesecond direction. In some embodiments, each of the first plurality ofcompound gratings may further include one fourth element between thesecond element and the at least two third elements. In someimplementations, each of the first plurality of compound gratings mayfurther include one gap disposed between the second element and thefourth element. In some implementations, the fourth element has a fourthwidth and the fourth width is greater than the second width. In someinstances, the at least two third elements may include 4 to 12 secondelements. In some instances, the first region of the overlay mark isadjacent to and aligned with the second region of the overlay mark alongthe second direction.

It is understood that additional processes may be performed before,during, or after the steps 1102-1104 of the method 1100. For reasons ofsimplicity, additional steps are not discussed herein in detail.

One embodiment of the present disclosure pertains to an integratedcircuit (IC) device. The IC device includes an overlay mark on asubstrate. The overlay mark includes an upper layer and a lower layerdisposed below the upper layer. The lower layer includes a firstplurality of compound gratings extending in a first direction anddisposed in a first region of the overlay mark, each of the firstplurality of compound gratings including one first element and at leasttwo second elements disposed on one side of the first element, and asecond plurality of compound gratings extending in the first directionand disposed in a second region of the overlay mark, each of the secondplurality of compound gratings including one third element and at leasttwo fourth elements disposed on one side of the third element. The firstelement and the third element each have a first width along a seconddirection perpendicular to the first direction. Each of the secondelements and each of the fourth elements has a second width along thesecond direction, the second width being smaller than the first width.The first plurality of compound gratings is a mirror image of the secondplurality of compound gratings.

In some embodiments, each of the first plurality of compound gratings oflower layer the further includes one fifth element. The at least twosecond elements are disposed between the first element and the fifthelement. In some implementations, each the first plurality of compoundgratings further includes one sixth element disposed between the atleast two fourth elements and the fifth element. In some instances, eachof the first plurality of compound gratings further includes one gapdisposed between the sixth element and the fifth element. In someembodiments, the fifth element has a third width and the sixth elementhas a fourth width. Each of the third width and the fourth width isgreater than the second width. In some implementations, the lower layerfurther includes a third plurality of compound gratings extending in thesecond direction and a fourth plurality of compound gratings extendingin the second direction. The third plurality of compound gratings is amirror image of the fourth plurality of compound gratings. In someembodiments, the upper layer includes a third plurality of gratings andthe third plurality of gratings is shifted with respect to the at leasttwo second elements in the second direction. In those embodiments, theupper layer further includes a fourth plurality of gratings and thefourth plurality of gratings is shifted with respect to the at least twofourth elements in the second direction. In some instances, the firstregion is adjacent to and aligned with the second region along thesecond direction.

Another embodiment of the present disclosure pertains to a method offabricating a semiconductor device. The method includes patterning anoverlay mark on a substrate and performing one or more semiconductorfabrication process using the overlay mark. The overlay mark includes anupper layer comprising a plurality of gratings extending in a firstdirection, and a lower layer disposed below the upper layer. The lowerlayer includes a first plurality of compound gratings extending in thefirst direction and disposed in a first region of the overlay mark, eachof the first plurality of compound gratings including one first element,one second element, and at least two third elements disposed between thefirst element and the second element; and a second plurality of compoundgratings extending the first direction and disposed in a second regionof the overlay mark. The first element has a first width along a seconddirection perpendicular to the first direction and each of the at leasttwo third elements has a second width along the second direction, thesecond width smaller than the first width. The second plurality ofcompound gratings is a mirror image of the first plurality of compoundgratings. The plurality of gratings is shifted with respect to the atleast two third elements in the second direction.

In some embodiments, a portion of the second plurality of compoundgratings is shifted with respect to the plurality of gratings in thesecond direction. In those embodiments, each of the first plurality ofcompound gratings further includes one fourth element and the fourthelement is between the second element and the at least two thirdelements. Also, in these embodiments, each of the first plurality ofcompound gratings further includes one gap disposed between the secondelement and the fourth element. Additionally, the fourth element has afourth width, and the fourth width is greater than the second width. Insome implementations, the lower layer further includes a third pluralityof compound gratings extending in the second direction and a fourthplurality of compound gratings extending in the second direction. Thethird plurality of compound gratings is a mirror image of the fourthplurality of compound gratings. In some instances, the first region isspaced apart from the second region.

Another embodiment of the present disclosure pertains to a method offabricating an overlay mark on a substrate. The method includes forminga first plurality of mandrel features at a pitch in a first region ofthe substrate, forming a second plurality of mandrel features at thepitch in a second region of the substrate such that the second pluralityof mandrel features comprises a mirror image of the first plurality ofmandrel features, forming spacers over sidewalls of the first pluralityof mandrel features and the second plurality of mandrel features, andremoving the first plurality of mandrel features and the secondplurality of mandrel features. In this embodiment, the first pluralityof mandrel features extends in a first direction. Each of the firstplurality of mandrel features includes one first mandrel and at leasttwo second mandrels disposed on a side of the first mandrel. The firstmandrel has a first width along a second direction perpendicular to thefirst direction and each of the second mandrels has a second width alongthe second direction. The first width greater than the second width.

In some embodiments, the first region is spaced apart from the secondregion. In some implementations, the first width is at least twice ofthe second width. In some instances, forming of the spacers oversidewalls of the first plurality of mandrel features and the secondplurality of mandrel features includes depositing spacer material overthe first plurality of mandrel features and the second plurality ofmandrel features, and planarizing the spacer material to expose topsurfaces of the first plurality of mandrel features and the secondplurality of mandrel features.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method of fabricating a semiconductor device,comprising: patterning an overlay mark on a substrate, wherein theoverlay mark includes: an upper layer comprising a plurality of gratingsextending in a first direction, and a lower layer disposed below theupper layer, the lower layer comprising: a first plurality of compoundgratings extending in the first direction and disposed in a first regionof the overlay mark, each of the first plurality of compound gratingsincluding one first element, one second element, and at least two thirdelements disposed between the first element and the second element, anda second plurality of compound gratings extending the first directionand disposed in a second region of the overlay mark; and performing oneor more semiconductor fabrication process using the overlay mark,wherein the first element has a first width along a second directionperpendicular to the first direction and each of the at least two thirdelements has a second width along the second direction, the second widthsmaller than the first width, wherein the second plurality of compoundgratings is a mirror image of the first plurality of compound gratings,wherein the plurality of gratings is shifted with respect to the atleast two third elements in the second direction.
 2. The method of claim1, wherein a portion of the second plurality of compound gratings isshifted with respect to the plurality of gratings in the seconddirection.
 3. The method of claim 2, wherein each of the first pluralityof compound gratings further includes one fourth element, the fourthelement being between the second element and the at least two thirdelements.
 4. The method of claim 3, wherein each of the first pluralityof compound gratings further includes one gap disposed between thesecond element and the fourth element.
 5. The method of claim 4, whereinthe fourth element has a fourth width, and wherein the fourth width isgreater than the second width.
 6. The method of claim 1, wherein thelower layer further comprising a third plurality of compound gratingsextending in the second direction and a fourth plurality of compoundgratings extending in the second direction, wherein the third pluralityof compound gratings is a mirror image of the fourth plurality ofcompound gratings.
 7. The method of claim 1, wherein the first region isspaced apart from the second region.
 8. A method of fabricating anoverlay mark on a substrate, comprising: forming a first plurality ofmandrel features at a pitch in a first region of the substrate, thefirst plurality of mandrel features extending in a first direction,wherein each of the first plurality of mandrel features comprises onefirst mandrel and at least two second mandrels disposed on a side of thefirst mandrel, wherein the first mandrel has a first width along asecond direction perpendicular to the first direction and each of thesecond mandrels has a second width along the second direction, the firstwidth greater than the second width; forming a second plurality ofmandrel features at the pitch in a second region of the substrate suchthat the second plurality of mandrel features comprises a mirror imageof the first plurality of mandrel features; forming spacers oversidewalls of the first plurality of mandrel features and the secondplurality of mandrel features; and removing the first plurality ofmandrel features and the second plurality of mandrel features.
 9. Themethod of claim 8, wherein the first region is spaced apart from thesecond region.
 10. The method of claim 8, wherein the first width is atleast twice of the second width.
 11. The method of claim 8, wherein theforming of the spacers over sidewalls of the first plurality of mandrelfeatures and the second plurality of mandrel features comprises:depositing spacer material over the first plurality of mandrel featuresand the second plurality of mandrel features; and planarizing the spacermaterial to expose top surfaces of the first plurality of mandrelfeatures and the second plurality of mandrel features.
 12. The method ofclaim 8, wherein the second plurality of mandrel features comprises themirror image of the first plurality of mandrel features with respect toa borderline between the first region and the second region, wherein thefirst mandrel is closer to the borderline than the at least two secondmandrels.
 13. A method, comprising: forming first compound gratings andsecond compound gratings in a first layer; forming third compoundgratings and fourth compound gratings in a second layer disposed overthe first layer; and measuring an alignment of the first layer and thesecond layer, wherein the third compound gratings are disposed over thefirst compound gratings and the fourth compound gratings are disposedover the second compound gratings, wherein the first compound gratingscomprise a first wide mandrel, a first gap, and a first plurality ofnarrow mandrels, wherein a width of the first wide mandrel is greaterthan a width of each of the first plurality of narrow mandrels, whereinthe second compound gratings comprise a second wide mandrel, a secondgap, and a second plurality of narrow mandrels, wherein a width of thesecond wide mandrel is greater than a width of each of the secondplurality of narrow mandrels.
 14. The method of claim 13, wherein themeasuring comprises use of a scatterometry machine.
 15. The method ofclaim 13, Wherein the first compound gratings are disposed within afirst region and the second compound gratings are disposed within asecond region, wherein the first compound gratings are a mirror image ofthe second compound gratings with respect to a borderline between thefirst region and the second region.
 16. The method of claim 15, whereinthe first wide mandrel is farther away from the borderline than thefirst plurality of narrow mandrels.
 17. The method of claim 13, whereina pitch of the first plurality of narrow mandrels is identical to apitch of the third compound gratings, wherein a pitch of the secondplurality of narrow mandrels is identical to a pitch of the fourthcompound gratings.
 18. The method of claim 13, wherein the thirdcompound gratings are shifted from the first plurality of narrowmandrels along a first direction by a distance, wherein the fourthcompound gratings are shifted from the second plurality of narrowmandrels along a second direction by the distance, wherein the firstdirection is opposite to the second direction.
 19. The method of claim13, wherein the first gap is spaced apart from the first wide mandrel bythe first plurality of narrow mandrels, wherein the second gap is spacedapart from the second wide mandrel by the second plurality of narrowmandrels.
 20. The method of claim 13, wherein the first wide mandrel,the first plurality of narrow mandrels, the second wide mandrel, and thesecond plurality of narrow mandrels are elongated and extend inparallel.